Magnetic particle inspection (MPI) processes are non-destructive methods for the detection of surface and sub-surface indications in ferromagnetic materials. These MPI processes make use of an externally applied magnetic field or electric current to produce a magnetic field through the material, and the principle that the magnetic flux from the field or current will leave the test article at the area of the indication. The presence of a surface or near surface indication in the material causes distortion in the magnetic flux through the indication, which in turn causes surface detectable leakage of the magnetic fields at the indication.
Magnetic particles are applied to the magnetized article and are attracted by the surface field in the area of the indication. The accumulation of these magnetic particles indicates the indication. Because indications can present a variable aspect relative to the direction of the applied field, typically two orthogonal orientations of the applied magnetic field, chordal and radial, are needed to inspect an elongated article such as a compressor blade. To produce a magnetic field orientation directed along the blade chordal direction, a current is applied in the platform to tip direction. To produce the magnetic field orientation in the radial direction, a surround coil of a MPI machine can be used.
The radially directed current to produce the chordal field can produce several problems or disadvantages. One disadvantage is that the current can produce blade damage at the contact regions. Arcing damage from the current can be severe at the blade tip where the contact region is small and the surface curved. For blades with a ‘pip tip’ or other protruding structure, arcing can be severe enough to produce a fire hazard when using hydrocarbon liquids to distribute the magnetic particle needed for the test. Another disadvantage is that the blade can become hot and damaged if the current is very high and multiple MPI current pulses are used.
The object of this disclosure is to provide an improved MPI inspection process that overcomes these and other disadvantages of known MPI processes.